There are cushioned bags commercially available today; however, such prior art packages are relatively bulky and heavy. For example, certain of such prior art cushioned bags are stuffed with shredded paper, or similar shredded or particulate masses of spongy filler material. Their structure is relatively complex, requiring a double-walled bag in order to contain the masses of particulate filler.
There are also cushioned shipping bags lined with microfoamed polyethylene material; however, such material is relatively slippery. It is often referred to in the packaging industry as having a "high slip," thereby allowing the contained item or items to slip and slide around at random within the polyethylene-lined package.
Although microfoamed polypropylene material is less expensive and lighter in weight than microfoamed polyethylene material, it has not previously been employed, insofar as we are aware, as the lining in cushioned shipping bags, because polypropylene microfoam material does not heat seal well with itself. Polypropylene material is sometimes referred to in the packaging industry as having a "low tack" when attempting to heat seal it with itself.
This problem of non-heat-sealability of polypropylene is advantageously overcome by coating the inner surfaces of the outer kraft paper layers with a thin coating of polyethylene. Then the intervening layers of polyproplyene material are burned away from between the kraft paper layers in the immediate heat seal regions and at the same time when the heat seals are being formed, so that these outer paper layers can be directly heat sealed together face-to-face through the inter-action of their polyethylene coatings. The heat energy for accomplishing this burn-away and heat sealing action is conducted from heated members through the respective kraft paper layers, and each seal is quickly set by fast-cooling action produced by cold chiller members applied to the exterior of the kraft paper layers in respective regions where the heated members were previously engaged thereon.
In the "burn-away" of the polypropylene microfoam material, this very low density material is quickly heated to a sufficiently high temperature that it melts and rapidly shrinks back upon itself, becoming absent from the region where the seal is being made.